Bad quality on zoom

I have developed an Android-app with ionic, and are using slidebox to show some ads, with the ability to pinch and zoom, like this:

<ion-slide-box on-slide-changed="slideChanged(index)">
<ion-slide ng-repeat="img in imgs" ng-if="img.filenavn.indexOf('ads') == -1">
	<ion-scroll zooming="true" overflow="none" min-zoom="1" max-zoom="3" direction="x" style="width: 100%; height: 100%;">
		<img src="{{img.filenavn}}" width="100%" zoomable />

If the app is used on a device like Samsung Galaxy SII, text on the zoomed image is pixelated and hard to read, even though the image file is twice the size of the devices resolution.

What am I doing wrong?

this was a known issue with zynga, and right now there doesn’t seem to be solution to the issue.

Thank you for your answer. So in order to get a decent zoom function, I have to implement an alternative?